(A) Field of the Invention
The present invention relates to a searching method. More particularly, the present invention relates to an autofocus searching method.
(B) Description of the Related Art
Recently, in the automatic optical inspection technology, fast autofocusing is one of the important steps in the optical measurement and inspection method. During the process of fabricating or inspecting the product, the use of fast image inspection methods has become mainstream, and in order to correctly inspect features of the product, it is necessary to obtain an image with high definition. The speed of manual inspection is relatively low, and the accuracy of inspection is easily influenced by human errors, so the reliability of the inspection is influenced also. Therefore, in order to achieve the objective of automatic optical inspection, it is desirable to obtain an autofocus algorithm in support of the optical image inspection method.
With the continuing development of semiconductor fabricating process, system miniaturization technology, and nanometer technology, the inspection equipment industry moves toward a combination of photoelectric technology and automation technology. In the field of miniaturization inspection, these new inspection technologies mostly utilize the non-destructive microscopic object lens method of optical inspection, which not only achieves quick inspection at the miniaturization level, but also further improves the degree of automation of the inspection procedure, combined with signal processing technology.
However, most optical microscopes commercially available on the market use manual focusing. For example, a target mark is added into the optical path as a focusing reference for the image embedment, such that the efficiency of the manual focusing is increased. However, the method is still affected by the quality of the operator, such that the quality of repeated accuracy may be inconsistent.
In order to achieve the objective of automatic optical inspection, the automatic focusing function is required. Inspection equipment with automatic focusing currently available on the market usually need to manually set a limited scope, and then the automatic focusing is performed within the limited scope, such that the efficiency of the automatic inspection equipment is greatly reduced. For example, for two-dimensional coordinate measuring machines commonly used in the industry, the focus searching method is to search for the focus step by step based on the fixed step size of the reference focal depth, and as the object lens moves away from the object to be inspected, longer times are needed for the focus searching.
In automatic focusing technology, the choice of focus searching process is a crucial factor that influences the automatic focusing speed. Focus searching processes can be divided into two categories, namely, stepwise focus searching and variable step focus searching. Stepwise focus searching has the lowest efficiency. Many autofocus processes employ the variable step method to reduce the searching steps and the searching time. However, searching is based on a single image index, such that the reference information of the variable step during the searching is very limited, and the achieved searching performance is also limited.
In Taiwan Patent Publication No. 5208451, a single point sensor is used to detect a position where the intensity of the light interference signal is maximal to serve as the optimal focus point. However, the hardware cost is relatively high, and point inspection is easily affected by noise and other environmental factors, such that the stability is not as desired. In Taiwan Patent Publication No. 486599, a two-stage process, including rough adjustment and fine adjustment, is used for focus searching. Although three evaluation values of images are taken into account, the step sizes are fixed for both rough adjustment and fine adjustment, so that it cannot quickly depart from the region far away from the focusing point. In addition, in Taiwan Patent Publication No. 571583, the focal depth is used as the reference for the focus-searching step, and it is necessary to set a focus searching range. This method cannot increase the searching speed through a large step manner, and autofocusing can only be performed in a short travel range.
The conventional focus searching method is limited to achieving a desirable searching effect close to the focal depth. When this method is applied to long travel focusing or to an object to be inspected that is far away from the focal depth, the value of focal depth must be used as the reference for the focus-searching step, and searching is performed to approach the object step-by-step, so as to avoid omitting images of good definition or colliding with the object. Because the focal depth varies according to the type and the magnification of the object lens, object lenses with shorter focal depth need longer times for focus searching, the efficiency of the automatic optical inspection equipment cannot be increased.
As to the microscopic object lens inspection method based on images, if the object to be inspected is manually moved to the focus position of the object lens, and if the moving speed is too high for the response of the human eye, it is easy to miss the image of good definition or the range of image interference.
In summary, either for machines currently available on the market or for published patents, focus searching for optical inspection still has shortcomings, and thus is unable to reach optimized efficiency.